Meet LUCA: Our Complex Ancestor

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The relative of all living things, dubbed the Last Universal
Common Ancestor (LUCA), was a sophisticated organism with a
complex structure recognizable as a cell, according to a new
paper in the journal Biology Direct.

Prior theories have held that this great-grandparent of all
living things was little more than a crude assemblage of
molecular parts, a chemical soup out of which evolution gradually
constructed more complex forms. This latest research suggests
that was not the case, although LUCA was not a multi-celled
creature.

“LUCA was probably a single celled organism because the
extracellular machinery needed for multicellularity was developed
in eukaryotes (organisms whose cells contain microstructures)
very late in evolution,” lead author Manfredo Seufferheld told
Discovery News.

“We do not know its appearance, but we hypothesize it looked very
much like an archaeum,” added Seufferheld, who is a professor of
crop sciences at the University of Illinois.

The archaea are a group of microorganisms that are somewhat
similar to, but genetically distinct from, bacteria.

The study builds on research into a once-overlooked feature of
microbial cells, a region with a high concentration of
polyphosphate.Seufferheld and colleagues Kyung Kim, James
Whitfield, Alejandro Valerio and Gustavo Caetano-Anollés made the
determinations after studying this region of cells which is a
type of energy currency.

Through genetic analysis, including the construction of “family
trees,” the researchers demonstrated how this polyphosphate
storage site actually represents the first known universal
organelle. Such specialized subunits, organelles, were once
thought to be absent from bacteria and their distantly related
microbial cousins, the archaea.

The evidence now indicates the organelle dates back to LUCA,
before the three main branches of the tree of life appeared. It
is therefore likely present in the three primary domains of
today’s Earthly life. These are bacteria, archaea and eukaryotes
(plants, animals, fungi, algae and everything else).

“We hypothesize that LUCA developed this organelle very early in
evolution so that it could have a sustainable source of energy
stored in its cellular make up,” Seufferheld explained, adding
that mitochondria (organelles functioning as cellular power
plants) “were later developed and replaced the major energetic
role” of the polyphosphate storage site when Earth became
oxygenated.

The findings add to what is already suspected about LUCA, thought
to have emerged about 3.8 billion years ago.

Nicolas Lartillot, a bio-informatics professor at the University
of Montreal, and his team also conducted genetic and tree of life
analyses. These suggest early life on Earth was composed of
ribonucleic acid (RNA) rather than DNA. RNA is particularly
sensitive to heat, so this provides yet another clue about LUCA.

Lartillot said that “our data suggests that LUCA was actually
sensitive to warmer temperatures and lived in a climate below 50
degrees.”

He said this negates the idea “that LUCA was a heat-loving or
hyperthermophilic organism. A bit like one of those weird
organisms living in the hot vents along the continental ridges
deep in the oceans today.”

Instead, LUCA may not have been able to exist under such extreme
heat conditions.

Whatever the temperature, it is believed LUCA was already at a
developmental stage that allowed for advanced protein synthesis.

“It is possible that having efficient cells capable of faster
replication through more efficient protein synthesis could have
triggered more cellular variants and the emergence of lineages,”
Caetano-Anolles said, thereby providing an explanation for how
LUCA may have given rise to the three main branches of the tree
of life.